Low Resistance Ground (LRG) on 480V System 1

[rockman7892]

Has anyone come across or have experience with LRG applied on a 480V system? I'm looking at a proposed project application that is calling for an 400/5 LRG applied on the neutral of (2) 4MVA 480V secondary transformers feeing a M-T-M 480V switchgear lineup.

I've always seen High Resistance Ground (HRG) systems applied at 480V level to allow continuous operation on ground fault and always understood LRG systems to be limited to MV systems? I believe I saw somewhere once that with an LRG applied on a 480V system, that under ground fault conditions most of the system voltage would drop across the resistor and may not provide enough voltage to drive ground fault to levels that can be selected by breaker trip units?

 

[GroovyGuy]

Nope, never seen a LRG applied to a 480Vac system.
What size of resistor are they proposing (ie 400A)?
Is this (400A) resistor to be continuously rated or short time (ie 30s)?
I think it would make more sense to install a 5A (ie HRG) continuously-rated resistor.
GG



"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[RRaghunath]

I too haven't come across LRG system for LV.
But, considering majority of faults are SLG type, it makes sense to limit the ground fault current especially when the source is so large as 4MVA.
The resistor ohmic rating will be quite small. Considering the current rating of 400A, it will be short time rated.
In case of HRG, the intent is to have reliable power supplies by not allowing the faulty feeder to trip automatically. With LRG, the intent is to minimise the damage at the point of fault.

 

[dpc]

Low-resistance grounding is definitely not recommended at 480 V. I don't believe it's allowed by the NEC. High-resistance grounding is allowed. LRG Grounding will cause issues with any downstream ground fault protection as well as fault clearing by fuses. And as you probably know, there can be no line-to-neutral loads when any type of resistance grounding is used.

 

[jraef]

I too have never seen it for LV before. The only advantage to LRG over solidly grounded is the reduction of the possibility of flashover to ground, which makes sense in MV and HV systems. I wonder if someone is thinking of this as a way to reduce Arc Flash risk? I've not heard of that before and I don't know if that would be valid, just speculating as to why someone is asking for something that doesn't seem to make sense. People have all kinds of weird ideas of how to get around the Arc Flash safety rules now.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Aradhana Ray]

Low-resistance grounding is definitely not recommended at 480 V. Where High-resistance groundings are allowed. It mainly consists of inserting a resistor into a three-phase generator, power transformer equal to limit the single line-to-ground fault current to a low value. Moreover, the ground should be of zero ohms resistance.